United Nations nuclear and health watchdogs have ignored evidence of deaths, cancers, mutations and other conditions after the Chernobyl accident, leading scientists and doctors have claimed in the run-up to the nuclear disaster's 20th anniversary next month.

In a series of reports about to be published, they will suggest that at least 30,000 people are expected to die of cancers linked directly to severe radiation exposure in 1986 and up to 500,000 people may have already died as a result of the world's worst environmental catastrophe.

But the UN's International Atomic Energy Agency (IAEA) and World Health Organisation say that only 50 deaths can be directly attributed to the disaster, and that, at most, 4,000 people may eventually die from the accident on April 26 1986.

They say only nine children have died of thyroid cancers in 20 years and that the majority of illnesses among the estimated 5 million people contaminated in the former Soviet Union are attributable to growing poverty and unhealthy lifestyles.

An IAEA spokesman said he was confident the UN figures were correct. "We have a wide scientific consensus of 100 leading scientists. When we see or hear of very high mortalities we can only lean back and question the legitimacy of the figures. Do they have qualified people? Are they responsible? If they have data that they think are excluded then they should send it."

The new estimates have been collated by researchers commissioned by European parliamentary groups, Greenpeace International and medical foundations in Britain, Germany, Ukraine, Scandinavia and elsewhere. They take into account more than 50 published scientific studies.

"At least 500,000 people - perhaps more - have already died out of the 2 million people who were officially classed as victims of Chernobyl in Ukraine," said Nikolai Omelyanets, deputy head of the National Commission for Radiation Protection in Ukraine. "[Studies show] that 34,499 people who took part in the clean-up of Chernobyl have died in the years since the catastrophe. The deaths of these people from cancers was nearly three times as high as in the rest of the population.

"We have found that infant mortality increased 20% to 30% because of chronic exposure to radiation after the accident. All this information has been ignored by the IAEA and WHO. We sent it to them in March last year and again in June. They've not said why they haven't accepted it."

Evgenia Stepanova, of the Ukrainian government's Scientific Centre for Radiation Medicine, said: "We're overwhelmed by thyroid cancers, leukaemias and genetic mutations that are not recorded in the WHO data and which were practically unknown 20 years ago."

The IAEA and WHO, however, say that apart from an increase in thyroid cancer in children there is no evidence of a large-scale impact on public health. "No increases in overall cancer incidence or mortality that could be associated with radiation exposure have been observed," said the agencies' report in September.

In the Rivne region of Ukraine, 310 miles west of Chernobyl, doctors say they are coming across an unusual rate of cancers and mutations. "In the 30 hospitals of our region we find that up to 30% of people who were in highly radiated areas have physical disorders, including heart and blood diseases, cancers and respiratory diseases. Nearly one in three of all the newborn babies have deformities, mostly internal," said Alexander Vewremchuk, of the Special Hospital for the Radiological Protection of the Population in Vilne.

Figures on the health effects of Chernobyl have always been disputed. Soviet authorities covered up many of the details at the time. The largest radiation doses were received by the 600,000 people involved in the clean-up, many drawn from army conscripts all over the Soviet Union.

Backstory

The worst nuclear accident in history took place on April 26 1986 when one of the four reactors at the Chernobyl complex 80 miles north of Kiev in Ukraine began to fail. Operators shut down the system, but a large chemical explosion followed a power surge and the 1,000-tonne cover blew off the top of the reactor. Design flaws in the cooling system were blamed for the accident, in which 31 people were killed immediately. The worst-affected area was Belarus, which took the brunt of the 4% of the 190 tonnes of uranium dioxide in the plant that escaped. Ukraine was also contaminated. Some 600,000 workers (mainly volunteers) who took part in recovery and clean-up operations were exposed to high levels of radiation; the Soviet government first suppressed news of the incident, but evacuated local people within a few days. Five million people were exposed to radiation in Belarus, Ukraine and Russia, and there was a dramatic increase in thyroid cancer among children living there.